KAFKA-16737: Enable unit tests for new consumer & cleanup TODOs

[lianetm]

KafkaConsumerTest contained lots of unit tests that were not enabled for the new consumer, with TODOs for verifying if they could be enabled. This PR enables all the unit tests that could be applied to the new consumer, and cleans up the TODOs.

The tests that remain being applied to the legacy consumer only cannot run for the new consumer, due to:

1. bugs in the new consumer. Only 8 tests in this case, for those I left TODOs just to reference the Jira I filed to make sure the tests are enabled as soon as the bugs are fixed.
2. the way the test is written or what it is testing (specific to the legacy consumer).

[lianetm]

Follow-up task KAFKA-16823 to refactor the test common functionality, and move out the unit tests that apply to the LegacyConsumer only, so we end up with KafkaConsumerTest for tests that apply to both consumers, AsyncKafkaConsumerTest (already exists for new consumer tests only) and LegacyKafkaConsumerTest (to be created for legacy consumer tests only). It will be addressed once the remaining bugs for the new consumer are fixed, so we can decide if the tests that are blocked should be moved out or not.

[lianetm]

Hey @lucasbru , could you take a look at this small one when you have a chance? Thanks!

--Edit--
Let me keep an eye on the build and play with it repeating it a bit. I'll ping you when it completes confidently. I had mixed unrelated build failures hiding some real ones with the tests that did not show locally. Thanks!

[lianetm]

Leaving as a draft for now as this seems to be affected by the timeout partial fix merged recently a98c9be. Will investigate and reopen this when ready.

[chia7712]

@lianetm sorry that I overlook this PR, but those tests covered by this PR are enable now (see #16370 and #16566). Maybe we can let this PR rest in peace 😄

[lianetm]

Hey @chia7712 , totally, I'll close this. It was only an initial exercise (helpful because it revealed bugs even though I didn't re-enabled the test that needed more work, I didn't have the bandwidth back then. Great that you and the other contributors could take on this!